Assembly of microtubules of flagella and chick brain are being studied in vivo and in vitro. Flagellar basal bodies and axonemes have been isolated from the alga Chlamydomonas and have been shown to act as assembly sites for chick brain microtubules. The microtubules assembly onto these structures shows directionality with distal addition favored. The directionality of brain microtubule assembly in vitro has been studied by electron microscopic autoradiography and shown to be principally uni-directional. The assembly of microtubules in vitro requires the presence of high molecular weight proteins in addition to tubulin. These proteins have some characteristics in common with the high molecular weight ATPase associated with flagellar tubules (Dynein). The Dynein also appears to be the protein which is limiting to flagellar regeneration in vivo in Chlamydomonas - e.g. the protein which must be synthesized in order for flagellar elongation to be completed. Other studies have been carried out on the role of flagellar membranes in the mating of plus and minus strains of Chlamydomonas - a process which is initiated by the sticking together of opposite mating types by the tips of their flagella.